Trim wheel very hard to turn

[DonMuncy]

5 hours ago, Shadrach said:

Don't get hung up on the shape of the airfoil. For the purpose of this discussion "flying" is due mainly to AOA (AOA is what makes a wing fly, Bernoulli makes it more efficient). 

 

Well said. I get perturbed reading that the wing shape is what causes the major part of lift. I like what Langeweische said in Stick and Rudder. "Airplanes go up by pushing air down".

[Shadrach]

18 hours ago, carusoam said:

1)  In the POH, Mooney uses a term called MAC in the WnB section.  The C's POH has it.  The R's POH has it and probably two sentences to describe it... (Insert what it means here?)  Cg is in front of Cl. During an ordinary stall, the main wing begins to stall first, lowering the nose to return to controlled flight.

 

Good morning,

-a-

Yes, I understand that, but I've recently been told that in flight, the CG is the fulcrum between the 2 opposing forces, as opposed to the CL which I previously was taught. If that's the case, would the plane not stall flat? Being that there are a number of engineers here on the board (to include you and Hank) I'm looking forward to a detailed and articulate response. Everything I find on the interwebs suggests that CL is the fulcrum in level flight.

Edited January 16, 2016 by Shadrach

[carusoam]

Detailed and articulate, we're going to have to wait for Hank's response...

The imaginary fulcrum is neither the Center of lift or the center of gravity.  See if I can make sense of this.  (It's been a while)

1) The Teater totter model indicates that the fulcrum is a fixed point that things rotate around.  It can supply as much force as needed to balance a herd of Marauder's GFs...

2) The flying airplane does not have this fixed point.

3) Somebody did mention the 'torque balance' above.  That and Coriolis accelerations are probably topics of dynamics.  310 hp comes with gobs of torque.  The balance part of WnB is balancing simple torques.  Force times distance (arm) = torque

4) Practical use of Coriolis accelerations is knowing to add right rudder when raising the nose for T/O.  Slightly different than the additional forces experienced from the rotating airstream caused by the prop.  Add left rudder trim during descent...

5) It is possible that the airplane model is more complex than the Teater-totter represents. It works pretty well for basic airmanship.

6) One thing we can count on, is that all the forces and torques balance each other out or there will be an acceleration in one direction or another, including rotations.  The reality of the stall/spin and why recovery comes first before adding power...

7) To calculate the imaginary fulcrum, Using the Teater-totter, aka balance beam model, we can determine where the center of rotation will be.  It is like doing a WnB but including the the two lift vectors with the center of gravity.  Select a point of reference like when doing a WnB.

8) the point of reference we use is not that critical. The part of the nose gear that is used is as good as anything else.

9) Similar topic: why are wing-tip tanks used to minimize the tendency to roll? Distributing mass far from the Cg helps.  To increase roll rates, move heavy things towards the Cg.

How was that?  I expect that I have made mistakes...

To do much better would (I think) require somebody like Scott in Kansas to give input.  He has the aviation engineering background.

Other complexities: Lift has some additional vector math quirks.  Drag is a non-linear function of airspeed. Imperfect Thrust is balanced by offsetting the engine mount.  And others...

Best regards,

-a-

[Shadrach]

Anthony,

I never said the fulcrum was fixed, I said it was the center of lift which changes with AOA as well as flap deployment. I am anxious to read Hank's response. It will blow my feeble aerodynamic mind. I'm not a trained engineer, but so much of what I thought about aerodynamics and stability rested on this principle.  When I pump the flaps down I can visualize the effective AOI changing as well as the CL moving aft as the airfoil changes with each pump of flaps. I envision the aircrafts downward pitch as a result of the fulcrum (CL) moving aft as the flaps are deployed. To find out that I've been thinking about it all wrong really has peaked my curiosity.

[carusoam]

Ross,

I am slowly catching the tracks that you are laying down...

1) I am having difficulty with the fulcrum part.  Because the teeter-totter really has one and airplane's don't.  It's the pointy object that the lever operates around. But, If planes did we could mathematically define it using the WnB technique including the centers of lift of both wings.  Essentially (I Think) we are discussing the center of rotation in the longitudinal axis.  Call that the fulcrum and we are talking about the same  thing.

2) we would include all the things that effect the lift and it's center like deploying speed brakes and flaps.

3) I would expect that a fare amount of work like this was done in the design and engineering stage.  Back in the day, may have been more trial and error than could go on today...

Other things...

4) The center of gravity moves about, as fuel and other liquids get used or expelled, and people move about in the cabin.  In Mooneys we slide the front seat back or lean the seat backs aft...

5) The Center of lift can also change as the flaps deploy.  Especially if they extend as they deploy.  That would include a change to wing area.

I think what we may have been discussing...

6) the amount of lift provided by the tail is the balancing force that offsets the smallish difference between the Center of lift (of the main wing) and the center of gravity of the whole plane.

7) as the Cg is moved towards the back of the normal envelope.  It approaches the Cg.  The tail force is adjusted out as required.

8) following this approach. When Cg is the same spot as the wing's Cl, the tail is providing no lift, and minimizes (lift induced) drag.  We get a feel for how much lift induces drag by how much speed we gain by sliding the seats back. Putting the tools in the baggage area. Has a similar effect in moving the Cg back, but it adds some less than appreciated weight as well.

9) in this condition, the stall would possibly be flat.  The nose may not definitively drop.  The airspeed may not be enough to generate lift from the tail to drive the nose down.  Recovery may require moving the Cg forward against gravity or increasing centrifugal forces.  Recovery may not occur before the ground is impacted.  This would be bad.

 

Follow up question:  When deploying Mooney flaps... What causes the nose to drop?

Related question: When going around under full power... What causes the nose to point skyward?

Am I getting closer?

Best regards,

-a-

[carusoam]

Ross,

I am slowly catching the tracks that you are laying down...

1) I am having difficulty with the fulcrum part.  Because the teeter-totter really has one and airplane's don't.  It's the pointy object that the lever operates around. But, If planes did we could mathematically define it using the WnB technique including the centers of lift of both wings.  Essentially (I Think) we are discussing the center of rotation in the longitudinal axis.  Call that the fulcrum and we are talking about the same  thing.

2) we would include all the things that effect the lift and it's center like deploying speed brakes and flaps.

3) I would expect that a fare amount of work like this was done in the design and engineering stage.  Back in the day, may have been more trial and error than could go on today...

Other things...

4) The center of gravity moves about, as fuel and other liquids get used or expelled, and people move about in the cabin.  In Mooneys we slide the front seat back or lean the seat backs aft...

5) The Center of lift can also change as the flaps deploy.  Especially if they extend as they deploy.  That would include a change to wing area.

I think what we may have been discussing...

6) the amount of lift provided by the tail is the balancing force that offsets the smallish difference between the Center of lift (of the main wing) and the center of gravity of the whole plane.

7) as the Cg is moved towards the back of the normal envelope.  It approaches the Cg.  The tail force is adjusted out as required.

8) following this approach. When Cg is the same spot as the wing's Cl, the tail is providing no lift, and minimizes (lift induced) drag.  We get a feel for how much lift induces drag by how much speed we gain by sliding the seats back. Putting the tools in the baggage area. Has a similar effect in moving the Cg back, but it adds some less than appreciated weight as well.

9) in this condition, the stall would possibly be flat.  The nose may not definitively drop.  The airspeed may not be enough to generate lift from the tail to drive the nose down.  Recovery may require moving the Cg forward against gravity or increasing centrifugal forces.  Recovery may not occur before the ground is impacted.  This would be bad.

 

Follow up question:  When deploying Mooney flaps... What causes the nose to drop?

Related question: When going around under full power... What causes the nose to strongly point skyward?

Don's Langweisch reference is certainly easier to understand from a practical point of view.

Am I getting closer?

Thanks for working with me on these topics.  

Best regards,

-a-

[carusoam]

I had some difficulty with the first post saving the finishing touches. Now there are two.

Sorry for the hassle,

-a-

[DonMuncy]

I hope I really don't need to understand all this in order to fly my plane.

[carusoam]

Understanding it is optional, Don.

putting it into practice is what counts.

Especially that go-around part.  Full flaps + full power = skyward view out the front window if you were not expecting quickly retrimming....

for new Mooney pilots, it can be a two-part surprise.  Coming in too fast forces a go-around.  They already fell behind the plane and then the cognitive challenges keep coming....

Better to have these discussions before they are needed.

To be clear I am only a PP, not a CFI.  My memory is improving.  It just ain't what it used to be...

Best regards,

-a-

[Hank]

I'm too tired to think about this, or even follow Anthony's posts above. I'll post a force diagram tomorrow. 

Putting tools in the back seat or baggage area DOES move the CG back. Lift is a force that acts at the CL. Weight is a force that acts at CG. For this discussion, we are breaking out tail downforce acting at some unknown point on the tail.

Lift can be assumed to act at the MAC (mean aerodynamic chord), which is in front of the CG. Weight can be treated as a point mass located at the CG.

the nose moves down with flap application because the wing profile is changing, and the angle of attack of the modified wing is higher. Remember that the wing tries to maintain the speed it is trimmed for? Raise the angle of attack (with Up elevator or Down Flaps), the plane will lower its nose to maintain the speed it is trimmed for, otherwise the plane would slow down.

The nose raises on a go around for two reasons:  1) airspeed is increasing, or you won't be "going around," you'll continue towards the ground, and lift is proportional to airspeed; 2) the prop is turning much faster, pushing air over the center section of the wing much faster than just before you crammed everything forward, and lift is proportional to airspeed.

and no, Don, none of us need to understand "Aerodynamics for Naval Aviators" to fly. Thankfully, as I've never read and don't own the book.

Edited January 16, 2016 by Hank

[Andy95W]

4 minutes ago, Hank said:

the nose moves down with flap application because the wing profile is changing, and the angle of attack of the modified wing is higher. Remember that the wing tries to maintain the speed it is trimmed for? Raise the angle of attack (with Up elevator or Down Flaps), the plane will lower its nose to maintain the speed it is trimmed for.

Do you know why this is different for different airplane types?  I have flown other low wing airplanes that adding flaps raises the nose.

[Hank]

Andy, it probably varies depending on the airfoil design, the amount of dihedral and the type of flap. We have barn door flaps, very simple, not at all like the fancy Fowler flaps on Cessnas that extend backwards making the chord longer, then as they continue to extend move out and down with a passage for airflow between the wing and the flap. Thus Fowler flaps are essentially a second small wing behind and below the big wing, and the aerodynamics becomes quite complex.

Our flaps don't extend, they are simply hinged to angle downwards, and even without the flap gap seals that some install, are pretty close to the rear surface of the wing. Our flaps are also pretty short, and at Full Down of 33° create much less drag than the Fowler flaps on a C172 at 30°, which may as well be speedbrakes.

[mike20papa]

The term "pitching moment" gets thrown around a lot when you read about the aerodynamic loads on a horiz. stab.  Pitching moment (the rotational force an airfoil develops as a result of lift & drag) increases as lift/speed increases, so the force the horiz. stab must develop varies with the wing's pitching moment from speed & lift.  But pitching moments vary from one type airfoil to the next.   I think the old Clark Y airfoils have higher pitching moments, laminar airfoils, much less - or at least the point at which they act relative to the aerodynamic lift center of the wing varies, so they are not so demanding to trim with every change in speed.

[Hank]

Nevertheless, I retrim after every speed change. But it's pretty easy. I suspect the OP has old, hard grease in his jack screw, or else not enough grease. My first annual, I put a lot of grease into the jack screw . . . Never has been hard to turn the wheel, though.

[Andy95W]

33 minutes ago, Hank said:

Our flaps don't extend, they are simply hinged to angle downwards, and even without the flap gap seals that some install, are pretty close to the rear surface of the wing. Our flaps are also pretty short, and at Full Down of 33° create much less drag than the Fowler flaps on a C172 at 30°, which may as well be speedbrakes.

Nice description, weird I'd never really looked at it like that.  The flap itself looks aerodynamic so you'd think there was something complicated going on.  Al Mooney probably could have just put a hinged panel on the bottom of the wing and had the same effect.

[Yetti]

How much weight on the tail, I am sticking with it depends on how the plane is loaded.

 

Separate conversation on go around: a lot of that wind that is being generated by the big fan is now passing over the horz stab too.    As for popping up fast it can also happen on second take off if you have somehow forgotten to reset trim to TO.    Not that I would know anything about that.  And a C172 will take off with full flaps and a full load.  Just don't expect to pitch up to 72 kts.  expect about 52kts and wondering why you are not going faster.   This is why it is good to know to fly the plane and not just do the numbers all the time.

[Yetti]

The flaps are thin, but they are very long.  They go about 2/3 of the wing.   So surface area is probably about the same as a 172.  That would be interested to measure the surface area of both.  Now they don't get to fowler flap disconnected with air flowing through the gap and they don't go to 44 degrees.    Some people say they are not doing much. I think half flaps does alot to lose altitude if you are high and make for a more stable approach.

[Shadrach]

There are many things that cause the nose to drop when flaps are deployed.  As I said earlier, the airfoil changes and with that change, the angle of incidence increases (angle between wing chord and longitudinal axis of the fuselage) and the center of lift moves aft. Additionally, depending on the airframe, flap deployment can affect airflow over the tail effecting a nose up, nose down or no pitching moment.  All of this is pretty easily understood.

What I am really curious about though is how and why these pitch changes apparently rotate around the CG and not CL (the very thing supporting the aircraft in flight).  I have yet to see or read anything that explains this.  I don't need it in detail here, just point me to the correct text or website and I'll read it myself.

[Yetti]

Magic and money is what keeps these things in the air.  Don't try and understand it, just believe

[Yetti]

62'' x 10" is the area of the flap

[Shadrach]

4 hours ago, bonal said:

I often wonder about the nose down when adding flaps what I think happens is the airflow is directed down and takes away airflow over the tail reducing the negative lift resulting in a shift of CG to the nose until trim can be adjusted to balance this out. When adding full power for a go around the tail is in a nose up trim and the increased airflow overcomes the flap setting resulting in a nose up. Perhaps with the high wing of the Cessna this effect is reduced going to full flaps on my 150 caused no change in pitch. Just a theory most likely not correct but seems reasonable to my left handed brain

Not in my F...Firewalled and climbing or Power off in descent deploying flaps gives nose down and raising them causes the aircraft to pitch up.

[N201MKTurbo]

2 hours ago, Shadrach said:

Not in my F...Firewalled and climbing or Power off in descent deploying flaps gives nose down and raising them causes the aircraft to pitch up.

Of course it does, the drag vector is below the CG. The air pushes on the flaps rotating the plane around the CG, pushing tha nose down.

[Hank]

6 hours ago, bonal said:

I often wonder about the nose down when adding flaps what I think happens is the airflow is directed down and takes away airflow over the tail reducing the negative lift resulting in a shift of CG to the nose until trim can be adjusted to balance this out. When adding full power for a go around the tail is in a nose up trim and the increased airflow overcomes the flap setting resulting in a nose up. Perhaps with the high wing of the Cessna this effect is reduced going to full flaps on my 150 caused no change in pitch. Just a theory most likely not correct but seems reasonable to my left handed brain

Airflow, power, etc., will not affect CG, as it is a weight-based calculation. No force will make the CG change, unless it is a force acting on unrestrained people/baggage inside the plane. All forces act on the CG. Lift acts at the Center of Lift, which affects the airplane as a force and distance from the CG; same for tail downforce.

[Shadrach]

18 hours ago, N201MKTurbo said:

Of course it does, the drag vector is below the CG. The air pushes on the flaps rotating the plane around the CG, pushing tha nose down.

I'm not sure if you're agreeing or disagreeing. I was responding to what I understood to mean in the sentence below that at high power setting the nose down tendency of flap deployment is negated by increased airflow.  I have not found that to be the case.

"When adding full power for a go around the tail is in a nose up trim and the increased airflow overcomes the flap setting resulting in a nose up."

[Yetti]

Flaps on the ramp beater c172   79"  x 19"